SM U-31 (Austria-Hungary)

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SM U-31 (Austria-Hungary) was a U-27 class submarine built for the Austro-Hungarian Navy. It was constructed by Ganz Danubius in Fiume, ordered on 12 October 1915, laid down on 4 July 1916, launched on 20 March 1917, and commissioned on 24 April 1917. U-31 was scrapped in 1920 after the war.

Commanders
- Franz Nejebsy: 24 April 1917 – 2 January 1918
- Hermann Rigele: 11 March – 31 October 1918

Service highlights
- U-31 served mainly in the Adriatic and the Mediterranean. After initial training, it patrolled the Adriatic coast of Italy and later the Mediterranean.
- On 25 June 1917, Nejebsy attacked an armed yacht but missed. After a port incident and engine problems, the boat reached Cattaro for repairs.
- On 10 August 1917, U-31 sank the Italian cargo ship Lealta (4,021 GRT). The next day, a destroyer rammed U-31, damaging its periscope and forcing an off-attack return. The boat reached Cattaro on 15 August.
- On 26 October 1917, U-31 sank in the harbor at Porto Bergudi for unknown reasons; it was raised the next day and repaired.
- In early 1918, Nejebsy left command and Hermann Rigele took over on 11 March. U-31 conducted several Mediterranean patrols from Pola and Cattaro.
- On 7 July 1918, U-31 sank the Italian sailing vessel Giuseppino Padre (67 GRT). In October, during Allied actions against Durazzo, U-31 launched torpedoes at HMS Weymouth, blowing its stern off and killing four sailors; the Weymouth was towed away after the attack. U-31 and U-29 faced depth-charge attacks from American forces and returned to Cattaro.
- After returning to port on 26 October 1918, U-31 remained in the area until the end of the war. It was awarded to France as a war reparation in 1920 and eventually scrapped at Bizerta.

Overall record
- Sunk: 2 merchant ships (total 4,088 GRT)
- Damaged: 1 warship

Specifications (U-27 class)
- Type: U-27-class submarine
- Displacement: 264 tons surfaced, 301 tons submerged
- Length: 121 ft 1 in (36.9 m); Beam: 14 ft 4 in (4.37 m); Draft: 12 ft 2 in (3.71 m)
- Propulsion: 2 diesel engines (270 bhp) for surface; 2 electric motors (280 shp) for submerged
- Speed: 9 knots surfaced; 7.5 knots submerged
- Complement: 23–24
- Armament: 2 bow torpedo tubes (4 torpedoes total); 75 mm/26 deck gun; 8 mm machine gun

U-31 was one of several Austro-Hungarian UB II-derived submarines built with German plans during World War I.